This invention relates to optical cross-connect equipment using optical amplifiers.
An optical amplifier consists of a single mode fiber having a diameter of several xcexcm to several hundreds of xcexcm, with a small quantity of erbium (the symbol of element: Er) etc. belonging to rare earth elements added into core part of the fiber.
By supplying a pumping light source, the optical amplifier can amplify tens of thousand times an optical signal input to this optical fiber.
The optical amplifier is called EDFA (Erbium doped fiber amplifier). The following description explains a case where an EDFA is used as an optical amplifier. An optical amplifier is generally classified into the following: an optical pre-amplifier having characteristic of low-noise amplification by improving responsivity, and an optical post-amplifier having high output and high saturation output property which enables long-haul transmission by further amplifying optical signals output to an optical transmission link, and so on.
In a backbone network, a wavelength multiplexing method in which a plurality of optical wavelengths are multiplexed has been in use. In such a network, optical cross-connect equipment (OXC) is used for switching optical transmission links at the unit of wavelength.
In case of either addition or removal of an optical transmission link, the number of multiplexed wavelengths in use in optical cross-connect equipment varies, which produces fluctuation in optical signal intensity (full light power) received in optical amplifiers.
When such fluctuation of optical signal intensity brings about optical surge in an optical amplifier or exceeds dynamic ranges in an optical amplifier or a receiver, transmission quality is deteriorated because of reduced S/N (signal to noise ratio) or increased error rate.
Namely, when an optical signal is not input because of a transmission link failure etc., an EDFA (optical amplifier) stores energy as an excited condition and emits a part of energy as an amplified spontaneous emission (ASE) i.e. emission of optical noise. If the transmission link is restored to produce abrupt input restoration of an optical signal, a pulse is generated with extremely high peak level.
This pulse is further amplified in an amplifier provided at a following stage, possibly to produce a damage to an optical signal reception device. Therefore, there is required a transmission system in which the intensity of optical signals to be input to an optical amplifier remains constant.
As mentioned above, while an optical amplifier is required for compensating loss in optical cross-connect equipment, noise is generated during optical amplification. To eliminate this, a wavelength selection means is introduced for selecting wavelengths respectively centered at each wavelength of a signal. Such optical amplifier generally provides a feedback control function such as ALC (automatic level control) to maintain output level constant for stabilizing optical output level. When an input light breaks, ALC is removed to produce a shutdown.
When the input light is restored, this shutdown is removed to resume ALC. It takes time however until light output is stabilized, because an output must gradually be changed to suppress generation of a surge.
A wavelength selection means normally supervises output light to perform feedback control for obtaining optimum wavelength positions. However, when a break occurs in input light, this feedback control is suspended until the input light is restored. This requires time until stabilized output light is restored.
It is an object of the present invention to provide optical cross-connect equipment using control light sources, enabling high-speed path switchover without producing an optical surge or decreasing transmission quality.
In conventional optical cross-connect equipment, the intensity of optical signals routed thereby varies in case of either a switchover of an optical transmission link caused by a failure etc., or installation or removal of a link. Such variation of intensity produces undesirable effect to transmission quality. In order to solve the above-mentioned problem, there is provided optical cross-connect equipment according to the invention, in which the following configuration is presented.
According to one aspect of the present invention, optical cross-connect equipment includes; an optical pre-amplifier; a routing portion for routing a signal light input through the pre-amplifier to a desired output port; a control light generator for generating a control light corresponding to the signal light; a selector for selecting either the input signal light received from the routing portion or the input control light to output; and a post-amplifier for amplifying the signal light or control light received from the selector.
Preferably, as an embodiment of the present invention, the signal light input through the pre-amplifier is a multi-wavelength signal light. The optical cross-connect equipment further includes a demultiplexer for splitting the multi-wavelength signal light. The routing portion routes the split signal light to a desired port. The split signal light is input to a selector. The control light input to the selector has a wavelength identical to the split signal light.
According to another aspect of the present invention, optical cross-connect equipment includes; an optical pre-amplifier; a demultiplexer which receives a multi-wavelength signal light for splitting into signal lights respectively having each plurality of wavelengths; and a routing portion for routing each signal light having each plurality of wavelengths split by the demultiplexer to a desired output port. The routing portion includes; a control light generator for generating control lights each corresponding to the signal light; a route switching means for switching signal lights having a plurality of wavelengths split by the demultiplexer to output; an optical amplification means for amplifying each signal light having each plurality of wavelengths output from the route switching means; a control light insertion portion for inserting the control light generated by the control light generator into an output of the optical amplification means; and a wavelength selection means for selecting an output of the control light insertion portion having a wavelength assigned to the wavelength selection means to output.
According to another aspect of the invention, preferably the control light insertion portion supplies, in advance to the route switching by said route switching means, a control light having a wavelength for use after a switchover to the wavelength selection means.
According to still another aspect of the invention, optical cross-connect equipment includes; an optical pre-amplifier; a demultiplexer which receives multi-wavelength signal light to output split-wavelength signal lights having each plurality of wavelengths; and a routing portion which routes each signal light having each plurality of wavelength split by the demultiplexer to each desired output port. The routing portion further includes; a control light generator for generating control lights corresponding to the respective signal lights; a route switching means for routing signal lights each having each plurality of wavelengths split by the demultiplexer for switching route to output; a control light insertion portion for inserting the control lights generated by the control light generator into the signal lights having a plurality of wavelength output from the route switching means; an optical amplifier for amplifying the output from the control light insertion portion; and a wavelength selection means for selecting an output of the optical amplification means having a wavelength assigned to the wavelength selection means to output.
According to a still further aspect of the invention, preferably the routing portion includes; a control light generator for generating control lights respectively corresponding to the signal lights; a route switching means for switching a route of the signal light having a plurality of wavelengths each split by the demultiplexer; a control light insertion portion connected at the preceding stage of the route switching means for inserting the control light generated by the control light generator into the signal light having a plurality of wavelengths; an optical amplification means for amplifying an output of the route switching means; a wavelength selection means for selecting an output of the optical amplification means having a wavelength assigned to the wavelength selection means to output.
Any further features of the present invention will become more apparent by the description on the embodiments referring to the accompanied charts and drawings.